High voltage electrolytic capacitors are employed as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density because it is desirable to minimize the overall size of the implanted device. This is particularly true of an implantable cardioverter defibrillator (“ICD”), also referred to as an implantable defibrillator, because the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume. While progress has been made in minimizing the overall size of the capacitors used in ICDs through the use of tantalum-based planar anodes that have a large internal surface area but a very small thickness so that they can be easily incorporated into ICDs, problems still exist. For instance, the planar anodes are anodized and are then sealed in a casing containing a highly conductive and generally corrosive liquid electrolyte solution, where an anode lead extends from the casing. Unfortunately, such wet capacitors can experience problems when the electrolyte leaks from the casing at the seal around the anode lead. For example, gases (e.g., hydrogen) may be evolved during operation, causing pressure to build inside the capacitor.
In light of the above, a gas-tight hermetic seal (e.g., metal-glass-metal hermetic seal) is often employed through which the anode lead can safely extend. However, the metal-glass-metal hermetic seal itself can sometimes become corroded by the liquid electrolyte and leak, and the small thickness of the casing makes designing a metal-glass-metal hermetic seal that can effectively prevent leakage of the electrolyte extremely difficult.
As such, a need currently exists for an improved hermetically sealed wet electrolytic capacitor for use in implantable medical devices, such as defibrillators.